Various types of electrochemical sensors to determine oxygen content in gases have been proposed. In such sensors which are particularly adapted for use with internal combustion engines, a housing, preferably of metal, holds the sensor element itself in rigid and sealed position. The sensor element as such includes an oxygen ion conductive solid electrolyte, such as zirconium dioxide. In one type of construction, and this is the type to which the present invention relates, the zirconium dioxide solid electrolyte is flat, in form of a plate. It may be self-supporting or positioned on a support plate or carrier. Two spaced film-type or layer-like porous electrodes are placed on the electrolyte, spaced from each other. At least one of those electrodes is exposed to the gas to be tested.
A simple type of sensor of the general construction is described in U.S. Pat. No. 4,157,282, Riddel. Two electrodes are applied to a plate-like solid electrolyte, both electrodes being exposed to the gas to be measured. The sensor operates in accordance with a potentiometric principle, that is, upon exposure of one electrode to the test gas and another electrode to a reference gas, a voltage will appear across the electrodes which is, respectively, indicative of presence or absence of oxygen in the gas to be tested. It has also been proposed to construct an oxygen sensor which has a carrier on which a layer-like solid electrolyte body is applied to which, in turn, an electrode is applied exposed to the exhaust gas and a second electrode to a material having a predetermined oxygen partial pressure. A layer-like heating element is also provided in German Patent Disclosure Document DE-OS No. 27 42 278, corresponding to U.S. Pat. No. 4,126,532, TAKAO et al.
Other types of sensors are polarographic sensors described, for example, in U.S. application Ser. No. 6,093 of Jan. 24, 1979, Dietz, assigned to the assignee of the present application, and now abandoned. This sensor has a reference electrode which is exposed to a reference gas--typically ambient air or, rather, the oxygen in ambient air; the measuring or sensing electrode is covered with a material forming a diffusion barrier to oxygen molecules derived from the gas to be tested. In a polarographic sensor, the output signal will be representative of the content of oxygen in the test gas if a voltage is applied across the electrodes. Another type of polarographic sensor, in which both electrodes are exposed to the measuring gas, and which includes a heating element as described in U.S. Pat. No. 3,691,023, Ruka et al.